A new Operation Inherent Resolve report has upped the official civilian death toll of the US-led aerial bombing campaign in Syria and Iraq by more than a third, confirming 484 civilian deaths – while activists claim the figure is several times higher.

“To date, based on information available, CJTF-OIR [Combined Joint Task Force] assesses that, it is more likely than not, at least 484 civilians have been unintentionally killed by Coalition strikes since the start of Operation Inherent Resolve,” a statement released on Friday reads.

The coalition analyzes each individual report and bases its official civilian death toll only on those it finds “credible.”

According to the coalition, in the period between August 2014 and April 2017, a total of 21,035 airstrikes were carried out against Islamic State (IS, formerly ISIS/ISIL). This figure breaks down into 44,330 separate engagements.

“Although the Coalition takes extraordinary efforts to strike military targets in a manner that minimizes the risk of civilian casualties, in some incidents casualties are unavoidable. Sixteen reports were assessed to be credible resulting in the unintentional death of 132 civilians,” the statement added.

The new report effectively upped the total civilian death toll by more than a third, as last month’s report acknowledged 352 civilian deaths.

The incident which contributed the most to the drastic increase in the official death toll was an airstrike carried out on March 17. A separate probe was launched into the incident, during which “101 civilians sheltered in the bottom floors of the structure, and four civilians in a neighboring structure were killed.”

A coalition report stated that air support was called in against two snipers targeting the Iraqi forces from a rooftop. The blame for the massacre, however, was shifted on to the terrorists, as the coalition claimed that the airstrike ignited ISIS-planted explosives, which led to secondary explosions and the collapse of the structure.

“The American people and the American military will never get used to civilian casualties. And we… will fight against that every way we can possibly bring our intelligence and our tactics to bear,” US Defense Secretary James Mattis told CBS last Sunday, while commenting on the March incident.

Mattis stated that the coalition has “already shifted from attrition tactics, where we shove [IS terrorists] from one position to another in Iraq and Syria” and pursued “annihilation tactics where we surround them.” While the new “tactics” inevitably leads to civilian casualties, they are needed for “accelerating the tempo” of the anti-terrorist campaign.

“Civilian casualties are a fact of life in this sort of situation,” Mattis said, adding that the coalition was doing “everything humanly possible consistent with military necessity” to “avoid civilian casualties at all costs.”

Independent researchers indicate that the numbers might be a magnitude higher.

“It’s like a scandalous understatement. I would ratchet up those figures by several orders of magnitude,” author and historian Gerald Horne told RT.

Calling civilian casualties a “fact of life” trivializes them and violates international laws, Horne said.

“Clearly, [Mattis’s statement] was a public relations disaster. That’s to say that expressing such an attitude basically bespeaks a cavalier approach to international law which look disdainfully at the idea of civilian casualties as some kind of collateral damage as Pentagon puts it. But, I’m afraid that’s the state of play as we speak.”